A small and light weight type electric/electronic apparatus, such as a notebook type computer for portability reasons, has become popular. Most of the electric/electronic apparatuses are capable of mounting a packaged battery (referred to as a battery pack, hereinafter) to operate them in locations where a commercial power source is not available. The battery pack mounted in the notebook type computer is charged through an AC adapter when the notebook type computer is connected to the commercial power source.
A charge/discharge operation of the battery pack is controlled by a charge controller provided within the notebook type computer. For example, the charge controller integrates a discharge current from the battery pack, and starts the charge operation to the battery pack when the charge controller detects that the remaining capacity of the battery pack has been reduced to a predetermined value, and the charge controller integrates a charge current to the battery pack at the charge operation and stops the charge operation when the charge controller detects that the battery pack is charged to a full charge state. The charged battery pack operates as a main power source of the notebook type computer to supply the power to each section of the computer when the notebook type computer is disconnected from the commercial power source.
It is known that a Lithium (Li) ion battery has been broadly used as the battery cell of the battery pack, since the Lithium ion battery generates a high energy density per weight and volume, and generally provides a reduction in size and weight of a portable type apparatus. However, the Lithium ion battery has a safety problem that its performance characteristic is degraded when it is charged by an overvoltage, and hence the design of the charging circuit and the overcharge protection circuit becomes important.
FIG. 4 shows a block diagram of a prior power supply circuit of the electric/electronic apparatus into which the battery pack is mounted. In the FIG. 4, a reference numeral 10 shows an AC adapter for converting a voltage of the commercial power source to a predetermined DC voltage, a reference numeral 11 shows the Lithium ion battery pack, a reference numeral 12 shows a charge device for charging the Lithium ion battery pack 11 to a full charged state when the DC voltage is supplied from the AC adapter 10, reference numerals 13 and 14 show a diode for preventing a reverse flow of current, a reference numeral 15 shows a DC/DC converter for converting a DC voltage from the AC adapter 10 or the Lithium ion battery pack 11 to a predetermined DC voltage (5V and 3.3V, for example) used in the electric/electronic apparatus to supply it to the apparatus, and a reference numeral 17 shows an OVP (Over Voltage Protection) circuit for preventing an overvoltage from being supplied to the Lithium ion battery pack 11. A reference numeral 18 shows a terminal for connecting the AC adapter 10 to the electric/electronic apparatus, and a reference numeral 19 shows a terminal for connecting the Lithium ion battery pack 11 to the electric/electronic apparatus. The terminal 18 includes a ground potential terminal, and the terminal 19 includes a voltage supply terminal, a ground potential terminal and a control terminal for performing the charge/discharge control, although these are not shown in the figure. The FIG. 5 shows one example of a circuit of the above OVP circuit 17.
In the FIG. 5., the OVP circuit 17 contains a SCR (Silicon Controlled Rectifier) 1, a voltage regulator diode ZD1, a resistor R1 and a fuse F1. A positive terminal of the battery pack 11 is connected to an anode of the SCR1 and a cathode of the ZD1, a cathode of the SCR1 is connected to a negative terminal of the battery pack 11 through the fuse F1, and an anode of the ZD1 is connected to the negative terminal of the battery pack 11 through the resistor R1 and the fuse F1. A gate terminal of the SCR1 is connected to a node A between the ZD1 and the resistor R1, and a voltage potential level at the node A between the ZD1 and the resistor R1 is applied to the gate terminal of the SCR1.
In the above construction, when an overvoltage is applied to the battery pack 11, the voltage regulator diode ZD1 turns on to raise the potential level at the node A by which the SCR1 turns on. When the SCR1 turns on, a discharge current I from the battery pack 11 flows through the fuse F1 to burn the fuse off (blow the fuse), so that the protection of the battery pack 11 from the overvoltage is performed.
Notwithstanding, the battery pack and the electric/electronic apparatus provided with such prior OVP circuit have caused the following problem. That is, although the overvoltage protection is reliably performed by burning the fuse F1 off, it is required for further use of the electric/electronic apparatus to replace the burned fuse F1 by a new fuse F1 since the fuse F1 is burned off each time that the OVP circuit is operated. Generally, the exchange of the fuse F1 is classified as a repair of the electric/electronic apparatus, and hence a process and a cost for sending the electric/electronic apparatus to a service center is required. The above overvoltage may be caused by other trouble, such as a solder ball or other electrically conductive particles accidentally contacted to the terminal portions, than the inherent described problem. Since such case was treated as the repair of the electric/electronic apparatus, it is a burden on users and the repair center. Also, it was required for burning the fuse F1 off to flow a large current from the battery pack, and hence the battery cell included in the battery pack is adversely affected.